The present invention relates to a composite writing instrument retractably accommodating two mechanical pencil units, for instance, two mechanical pencil units having different lead diameters, within a single tubular casing. In particular this composite writing instrument is characterized in that it comprises a lower tubular casing accommodating therewithin the two mechanical pencil units spaced by about 180.degree., and an upper tubular casing enclosing a cam mechanism for effecting a reciprocal rotational motion of about 180.degree. around the axis of the lower tubular casing as well as engaging both mechanical pencil units for shifting the units alternately in the axial direction. The upper tubular casing is rotated around the axis for advancing one of the pencil units to a writing position where it projects out of the lower end of the lower tubular casing and retracting the other of the two pencil units to a retracted position within the lower tubular casing, and vice versa. An operation for feeding the lead of the pencil unit disposed in the writing position can be effected by reciprocating the upper tubular casing in the axial direction.
There has heretofore been known such a writing instrument which is provided with a lower tubular casing accommodating therewithin two different colored ball point pen units and an upper tubular casing enclosing a cam mechanism adapted to be engaged with said two ball point pen units, the upper tubular casing being reciprocally rotatable around the axis of the lower tubular casing through a certain angle for shifting the two pens units to a writing position or a retracted position. In this prior art structure of the writing instrument, however, the upper tubular casing is unable to be shifted in the axial direction of the lower tubular casing, so that even if a mechanical pencil unit is substituted for a ball point pen unit, the feeding action of the lead by and axial manipulation of the upper tubular casing cannot be effected.